There are various kinds of systems for detecting the presence or absence of an object and changing the control when the presence of an object is detected. For example, when the object is a raindrop, the control of a window wiper control apparatus of an automobile windshield is changed circumstantially when the weather changes and it starts raining. An important problem to facilitate the convenience of this window wiper control apparatus includes a development of a rain sensor for detecting whether or not it is raining. Hereinafter, as a conventional object sensor, a conventional rain sensor for detecting a raindrop as an object on a windshield of an automobile will be explained.
In the case of a generally available manually-operated window wiper, when a driver notices that it starts raining, he/she has to take into account the running state of an automobile and the change in the quantity of raindrops on a windshield and to turn a switch of the window wiper from off to on in order to secure the sight of view through the windshield necessary to drive a car. In order to ease such inconvenience of manual switching operation of the window wiper, a rain sensor is provided to detect the presence of an object such as a raindrop on a sensing surface of a windshield and to judge the necessity to operate the window wiper.
As a conventional rain sensor, in accordance with methods for detecting a raindrop, various sensors have been proposed and a reflected light detection type rain sensor, etc. is known. FIG. 25 is a view for simply explaining a principle of detecting a raindrop by the conventional reflected light detection type rain sensor. In FIG. 25, reference numeral 1000 denotes a windshield of an automobile. For convenience of explanation, the upper space of the windshield 1000 is the inside a car, that is, a space of a driver's side and the lower space is the outside. Reference numeral 1010 denotes a light source; 1020 denotes a prism; 1030 denotes a prism for leading out a reflected light from the inside of the windshield; 1040 denotes a lens, 1050 denotes a PD (photo-detector) as a light-receiving element; and 1110 denotes a sensing surface. Reference numeral 1120 denotes a raindrop that is present on the sensing surface. From the light source 1010, a bundle of rays of light having an extension capable of covering the entire sensing surface is emitted. Reference numeral 1130 denotes a path of a ray of light incident in a portion of the sensing surface 1110 on which the raindrop is present. Reference numeral 1140, other than the ray of light 1130, denotes paths of light incident in the sensing surface 1110 on which no raindrop is present.
In the reflected light detection type rain sensor, it is important to adjust an angle at which each element is attached and a material (in particular, a refractive index of a material). The principle of sensing a raindrop will be explained briefly. Light incident in the portion on which a raindrop is present on the sensing surface leaves outward from the external surface of the windshield 1000 because the total reflection condition is not satisfied. On the other hand, light incident in the portion on which no raindrop is present is totally reflected by the external surface of the windshield 1000 because the total reflection condition is satisfied and thus the difference in the strength of the reflected light is detected.
Therefore, for the light source 1010 and the prism 1020, an angle and material are selected so that the incident condition in which emitted light is incident inside of the windshield 1000 is satisfied. The angle of the light source is selected so that emitted light is totally reflected by the external surface of the windshield 1000. Furthermore, due to the change in the refractive index by the presence of a raindrop, the incident angle of light is selected so that the states are switched between the state in which the total reflection condition is satisfied and the state in which the total reflection is not satisfied.
Also for the prism 1030, a material and angle are selected so that an emitting condition is satisfied in which reflected light can be emitted to the outside of the windshield 1000, that is, the total reflection condition is not satisfied. For the lens 1040 and the light-receiving element 1050, an angle and distance are adjusted so that light incident in the lens 1040 is converged in a portion of the sensor of the light-receiving element 1050.
Note here that these elements 1010 to 1050 can be attached to places other than in the windshield 1000, for example, on a hood or a roof. However, since the subject to be detected is a state of the windshield 1000, these elements are preferably attached on a portion to be wiped off by a wiper, which is a part of the windshield 1000. Furthermore, it is preferable that such elements are attached so as to not narrow a driver's sight of view. For example, it is preferable that such elements are attached to a portion of the windshield in which a rearview mirror is provided and the sight of view is already hindered.
The operation of the above-mentioned conventional reflected light detection type rain sensor will be explained briefly. A bundle of rays of light emitted from the light source 1010 is led to the inside of the windshield 1000 and is incident in the entire surface of the sensing surface 1110. Now, on the sensing surface 1110, a raindrop 1120 is assumed to be present. The ray of light 1130 incident in a portion on which the raindrop 1120 is present among the rays of light incident on the sensing surface 1110 leaves outward from an external surface on the windshield 1000 because the total reflection condition is not satisfied due to the presence of the raindrop, which has a refractive index n of about 1.3. Thus, this ray of light is not detected by the light-receiving element 1050. On the other hand, among the rays of light incident in the sensing surface 1110, rays of light 1140 incident in a portion on which no raindrop is present among the rays of light incident on the sensing surface on the windshield 1000 are totally reflected because the total reflection condition is satisfied by the presence of air having a refractive index n of 1. Light that is totally reflected by the sensing surface 1110 enters inside of the car without being totally reflected at the portion in which the prism 1030 is present on the internal surface of the windshield 1000 facing inside the car. The emitted light is converged on a portion of the light sensor on the light-receiving element 1050 by the lens 1040.
In this way, the quantity of light detected by the light-receiving element 1050 decreases when the raindrop 1120 is present, and the quantity of the received light decreases with the increase in the area of the sensing surface 1110 covered with the raindrop 1120. By detecting the change in the light quantity, the presence of a raindrop on the sensing surface 1110 is detected. The explanation mentioned above is a principle of the detection of raindrops by using a conventional reflected light detection type rain sensor.
Note here that each type of rain sensor has a configuration in which raindrop detection signals are output when the change of signals are detected. The raindrop detection signals from the rain sensor are input to a control unit of a window wiper and the control of the predetermined window wiper is carried out, triggered by the input of raindrop detection signals.
However, the above-mentioned conventional rain sensor has the following problems.
There was a problem in that a conventional rain sensor could not detect a flashing phenomenon. Depending upon various factors such as kinds of objects, water repelling ability of a windshield, driving and running state of a car, and the like, even if the object is the same, the shapes thereof become different. It is known that due to such a change in the shape of the raindrop, a flashing phenomenon occurs when extraneous light is incident on the raindrop from a source external to the windshield 1000. The flashing phenomenon is a phenomenon that is caused by the shape of the object when the emitted light from a light source provided at the outside of the windshield 1000 was a light strength that is relatively stronger than the quantity of light from the light source 1010 that is incident in the object on the windshield. If the occurrence of this flashing phenomenon can be detected, the surface of the windshield can be wiped off by a wiper quickly and the object can be removed so as to ease the flashing phenomenon.